1. Field of the Invention
The present invention relates to a clutch device for an telescopic power antenna mounted on automobiles, etc.
2. Prior Art
As is well known, conventional clutches of this type are designed as follows: A rotary force is transmitted by causing one side of a main driving clutch plate, which is connected to a motor side, to press against one side of a driven clutch plate which is connected to an antenna side, so that the antenna element (which constitutes the load) is extended and retracted. When the extension or retraction of the antenna element is completed, the slipping action of the clutch plates allows the motor to rotate alone until the power supplied by a motor is switched off by a limit switch. In this way, motor locking is prevented.
In the conventional clutches designed as described above, inconsistency in the clutch plate pressing contact force tends to occur. Accordingly, it is difficult to obtain a stable clutch function over a long period of time. Furthermore, the thickness of the clutch in the axial direction of the clutch plates tends to become large, so that the overall size of the clutch is increased. In an attempt to eliminate such drawbacks, devices have been proposed in which the main driving clutch plate and driven clutch plate are positioned to face each other in a radial direction, and a means for generating a pressing contact force between the inner circumferential surface of one clutch plate and the outer circumferential surface of the other clutch plate is installed so that rotary force can be transmitted.
FIGS. 3(a) and 3(b) illustrate a conventional device which is constructed as described above. A rotary shaft C, which is installed inside a housing (not shown) of antenna drive mechanism so as to be able to rotate, is shown at the center of each of the FIGS. An electrically driven telescopic antenna clutch mechanism A is mounted on this rotary shaft C. The electrically driven telescopic antenna clutch mechanism A is made up with a main driving clutch plate 1, a driven clutch plate 2 and a pressing contact mechanism 3. The main driving clutch plate 1 is formed of a synthetic resin and has a gear la formed on its outer circumferential surface for receiving a driving force of a motor (not shown). Similarly, the driven clutch plate 2 is formed of a synthetic resin and has a drive gear 2a on its outer circumferential surface for transmitting a driving force to antenna-driving rope. The main driving clutch plate 1 is mounted on the rotary shaft C so that the plate 1 is free to rotate relative to the shaft C. On the other hand, the driven clutch plate 2 is mounted on the rotary shaft C so that it cannot rotate relative to the shaft C. The pressing contact mechanism 3 is designed as follows: an engaging pin 4 which engages with an engaging groove 2c formed in inner surface 2b of the circumferential wall of the driven clutch plate 2; a first pushing block 5a having a tip which pushes the engaging pin 4 against the inner circumferential surface 2b of the circumferential wall; a second pushing block 5b installed on the opposite side of the mechanism so that it forms a pair with the first pushing block 5a, the second pushing block 5b having a tip caused to be in sliding contact with the inner surface 2b of the circumferential wall; and coil springs 6a and 6b, which are elastically compressed between the rear sides of the first and second pushing blocks 5a and 5b, are installed in a housing recess formed in one side of the main driving clutch plate 1.
When the main driving clutch plate 1 is rotated by a motor in a conventional clutch for use in electrically driven telescopic antennas designed as described above, the rotary force is transmitted to the driven clutch plate 2 via the pin 4 and engaging groove 2c. Accordingly, the driven clutch plate 2 rotates together with the main driving clutch plate 1, so that the antenna element is extended or retracted.
When the extension or retraction of the antenna element is completed, the driven clutch plate 2 stops its rotation. Since the rotation-stopping force of the driven clutch plate 2 naturally exceeds the coupling force between the main driving clutch plate 1 and the driven clutch plate 2, the pin 4 is released from the engaging groove 2c and slides on the inner circumferential surface 2b of the driven clutch plate 2. In other words, the clutch disengages, so that the main driving clutch plate 1 idles by itself.
In a conventional clutch for use in electrically driven telescopic antennas constructed as above, the following problems arise: when the main driving clutch plate 1 idles upon completion of the extension or retraction of the antenna element, a large impact noise is generated by the engagement and disengagement of the engaging pin 4 with the engaging groove 2c. When the engaging pin 4 is disengaged from the engaging groove 2c, the frictional force generated between the main driving clutch plate 1 and the driven clutch plate 2 consists almost entirely of the frictional force between the tip of the pushing part 5b and the inner surface 2b of the circumferential wall of the driven clutch plate 2. As a result, slipping occurs between the main driving clutch plate 1 and the driven clutch plate 2, creating a situation in which the antenna element might drop by its own weight.